I noticed that for AES256, 36 bytes should be supplied as a key (instead of 32 bytes).
What is the origin of the extra 4 bytes? And in general, what is the format that should be given for such decryption key?

AES-GCM-ESP with a 256 bit key
The KEYMAT requested for each AES GCM key is 36 octets. The first
32 octets are the 256-bit AES key, and the remaining four octets
are used as the salt value in the nonce.